Composites have many advantages for use as aircraft structural materials including their high specific strength and stiffness, resistance to damage by fatigue loading and resistance to corrosion. Multiple industries, most notably the aircraft industry, continue to increase their use of composite materials, most noteworthy in the arena of principle structural elements. This expanded use, coupled with difficulties associated with damage tolerance analysis of composites, has placed greater emphasis on the application of accurate nondestructive inspection (NDI) methods.
In addition, advances in structural adhesives have permitted engineers to contemplate the use of bonded joints in areas that have long been dominated by mechanical fasteners and welds. The deployment of bonded joints generally requires the use of sensitive nondestructive inspection techniques to ensure the continued integrity of the bond joint. Adhesive bonding is being used in the manufacture of high performance components such as wind turbine blades, civil structures and aircraft.
These components may be formed of highly-attenuative materials of varying thickness. Such high performance applications require highly sensitive flaw detection. However, current quality control relies primarily on the robust control of the production process, the adhesive preparation and/or the careful application of materials. However, there is no practical method currently available to assess the overall quality of the composite structure or adhesive joints with sufficient sensitivity.
Additionally, installed adhesive bonded structures, such as wind turbine blades and aircraft components may incur failure or degradation. A typical aircraft can experience over 2,000 fatigue cycles (cabin pressurizations) and many more flight hours in a single year. Wind turbine blades can experience millions of fatigue cycles in a single year of operation. The unavoidable by-product of this use is that flaws develop throughout the structure's skin and substructure elements. The main causes of structural failure in these components are environmental degradation, adhesive disbonds, interply delaminations, and subsurface fiber fracture due to impact. When these types of damage occur, they may lead to catastrophic failures. By their nature, they occur at an interface and are, therefore, always hidden. A combination of fatigue loads and other environmental weathering effects can combine to initiate these types of flaws. A periodic inspection of composites for disbonds and delaminations (from fabrication, installation, fatigue, or impact damage) is essential to assure the successful operation of the structure over time. The interactions at the bond interface are extremely complex, with the result that the strength of the bond is difficult to predict or measure. Even a partial disbond may compromise the integrity of the structural assembly. Therefore, it is necessary to detect all areas of disbonding or delamination before joint failures can occur.
As the commercial airline industry responds to calls for the ensured airworthiness of global airline fleets, inspection reliability is of growing importance. The development and application of new Nondestructive Inspection (NDI) techniques needs to keep pace with the growing understanding of aircraft structural aging phenomena. Ultrasonic inspection or testing is a nondestructive method in which beams of high frequency sound waves are introduced into materials for the detection of surface and subsurface flaws in the material. In ultrasonic pulse-echo inspections, short bursts of ultrasonic energy are interjected into a test piece at regular intervals of time. In most pulse-echo systems, a single transducer acts alternately as the sending and receiving transducer. Sometimes it is advantageous to use separate sending and receiving transducers for pulse-echo inspection.
The sound waves, normally at frequencies between 0.1 and 25 MHz, travel from the transducer through a water column within the inspection device and into the material with some attendant loss of energy (attenuation) and are reflected at interfaces within the test article. The water column between the ultrasonic transducer and the inspection surface produces the signal coupling needed to interrogate the test article.
The reflected beam is displayed and then analyzed to define the presence and location of flaws. The degree of reflection depends largely on the physical state of the materials forming the interface. Fracture, delaminations, shrinkage cavities, pores, disbonds, and other discontinuities that produce reflective interfaces can be detected. Complete reflection, partial reflection, scattering or other detectable effect on the ultrasonic waves can be used as the basis of flaw detection. In addition to wave reflection, other variations in the wave, which can be monitored, include: time of transit through the structure to be inspected, attenuation, and features of the spectral response.
In traditional ultrasonic inspection devices, the height of the water column (the distance that the sound waves travel from the transducer to the test article or device signal travel distance) is fixed. Depending on the total travel distance (the sum of the fixed transducer travel distance and the penetration depth), the reflected signal may include noise and/or masking signals from harmonics or other undesirable reflections within the part. An operator may use a single or several ultrasonic inspection devices having a selected fixed transducer travel distance or distances to try to minimize noise and/or avoid the presence of masking signals created by signal harmonics or other undesirable reflections.
However, these fixed transducer standoff devices are not capable of adjusting the water path distance to shift the undesirable masking signals away from the true signals of interest. Often, parts being inspected do not have a fixed thickness, where the signals of interest vary and can at certain thicknesses be masked by front surface signal multiples. When inspecting parts of varying thickness, it is necessary to adjust the distance between the ultrasonic transducer and the surface of the test article so that the unneeded harmonic signals do not interfere with the signals of interest. In addition, it is necessary to suspend a water column between the ultrasonic transducer and the inspection surface in order to produce the signal coupling needed to interrogate the test article.
Thus, there is a need for ultrasonic inspection device that can vary the device signal travel distance.